Prototype 2 is a brilliant game that has to contend with its predecessor, an equally brilliant game. The original Prototype was extremely fun but ultimately suffered from a very limited world to explore. Prototype 2 is slightly less limited in scope but also does not add much to the scale of the gameplay experience. Is Prototype 2 worth buying or is it just a rip-off of its predecessor? I think it is a little of both.
Prototype was already a great game. It offered a lot of very interesting meta-games along with a fairly engrossing but somewhat short story arc. The pace of the game was perfect, and the menu system and control scheme were well designed. Prototype 2 takes all of those elements add somehow makes a more engrossing and more addictive experience.
As far as I can tell, the only part of Prototype 2 that is lacking is the meta-games. They seem to be tied into a DLC system, Radnet, that requires an internet connection. However, there are still some meta-games mixed into the gameplay of the story arc. Some of these meta-games are simple grab-and-dash objectives. Others are stealth missions. Prototype 2 does a better job of integrating meta-games into the story missions, but it does not offer much to do after completing the mission objectives.
If you liked Prototype, you will like the sequel. Overall, it is a simple continuation of the story and does not offer long term entertainment. It is worth renting, but it is not a game most people would return to after completing. If you can buy it for $20 or less, you will get your money's worth playing through the game just once.